JP2622537B2 - Measuring instrument for detecting alcohol content in air-fuel mixture - Google Patents

Description

The present invention is a measuring instrument for detecting the alcohol content of an air-fuel mixture by measuring the capacitance and conductance of a condenser, wherein the condenser is formed by a part of a casing wall. The measuring device includes a first electrode and a second electrode formed by a sensor element exposed to a flow of an air-fuel mixture. The measuring device is provided with a measuring circuit, and the measuring circuit is electrically connected to the capacitor. The present invention further relates to a measuring instrument having an oscillator, wherein the capacitance of the capacitor is determined from the frequency of the oscillator.

EP 0 335 168 describes alcohols
A method for driving an internal combustion engine driven by a gasoline mixture is described. The measurement of the alcohol content of the supplied mixture is used for pre-control of the supplied mixture.
The alcohol content of the air-fuel mixture is detected by a common measurement circuit by measuring the capacitance and the loss resistance (hereinafter, referred to as conductance) of the measurement device through which the air-fuel mixture flows. This measuring circuit can oscillate, the frequency of which is evaluated as a measure of the capacitance. The measuring device is preferably used as a frequency determining element of an oscillator.
The frequency of the oscillator can be changed by the parallel connection of another capacitor or by a switchable delay.

The oscillator operates at a high frequency and emits a signal approximately determined by the capacitance of the measuring device. This signal is also dependent on the conductance which represents the loss of the capacitor. Despite the use of very high-speed components, the transit delay of the signal from the input of the oscillator to its output can no longer be neglected. Depending on the manufacturing tolerances of the components, this delay time can vary.

The travel delay further depends on the temperature and the supply voltage,
In addition, high costs are required to detect the capacitance of the measuring device due to its aging.

The object of the present invention is to reduce the capacitance of the measuring device,
It is an object of the invention to provide a measuring device as described at the outset, which can be detected in view of component tolerances.

According to the invention, the input of the oscillator is connected to a control circuit, from which at least three control voltages of different magnitudes are supplied, and the output of the oscillator is connected to an evaluation circuit. The problem is solved by configuring the evaluation circuit such that the oscillator frequency adjusted for each of the control voltages is measured, and the capacitance and conductance of the capacitor are determined from the measured oscillator frequency.

An advantage of the present invention is that the measuring device is composed of inexpensive and simple components. The measuring instrument has a measuring device and a measuring circuit. The measurement circuit includes an oscillator and a control circuit. The oscillator has a comparator, which is connected to the RC network only. The measuring device is part of the RC network. The capacitance and conductance of the measuring device can be detected with only one sensor element.

The oscillator oscillates at a frequency that is changed by the control circuit within a measurement cycle. The frequency is evaluated to detect capacitance and conductance. As the control circuit, a commercially available gate or an integrated circuit having a semiconductor circuit is used. This gate has a tri-state output. The gate and the integrated circuit are connected only to the resistor network. Measurement circuit is application specific integrated circuit (ASIC)
It can also be realized as.

Two embodiments of the present invention are shown in the drawings and are described in detail below.

1 is a cross-sectional view of a measuring device for detecting the alcohol content of an air-fuel mixture, FIG. 2 is a circuit diagram of a measuring circuit of the measuring device of FIG. 1, and FIGS. 3A and 3B are FIGS. FIG. 4 is a diagram of a measuring circuit of another embodiment of the measuring device of FIG. 1, and FIG. 5 is a diagram showing an alcohol content and a relative amount of the mixture at a mixture temperature T = 20 ° C. graph showing the relationship between the dielectric constant epsilon _r, 6, for mixture with different alcohol content, it is a graph showing the relative permittivity epsilon _r which depends on the temperature T.

The measuring device (FIG. 1) for detecting the alcohol content of the mixture has a measuring device and a measuring circuit 1. The measuring device has a housing 2 and, for example, two sensor elements 3 and 4. The mixture containing alcohol and gasoline flows in the fuel channel in the direction of the arrow through the casing 2. The sensor elements 3 and 4 are glass-mounted on the intermediate plate 6 and are electrically connected to the measuring circuit 1 via wires 7. The first sensor element 3 is provided with a temperature sensor 8 for measuring an air-fuel mixture temperature. Casing 2
Are closed by a bottom plate 9.

Part of a wall of the casing 2 forms a first electrode, the sensor element 3 forms a second electrode of the capacitor C _p. The capacitance of this capacitor is measured. Air mixture flows through the sensor element 3 4, whereby to form a dielectric of the capacitor C _p. The relative dielectric constant epsilon _r of the mixture from the capacitance of the capacitor C _p is calculated.

The mixture comprises alcohol and gasoline volume components.
Alcohol and gasoline have different dielectric constants. The relationship between the relative permittivity epsilon _r and alcohol percentage by volume of the mixture,
FIG. 5 shows the case where the mixture temperature T = 20 ° C. The relationship between the relative permittivity ε _r and the mixture temperature T in degrees Celsius (° C.) is shown in FIG. 6 for different volume percentages of alcohol. Thus knowing the relative permittivity epsilon _r of the mixture, it is possible to determine the alcohol content.

The capacitance of the capacitor C _p is measured by the first sensor element 3 (Figure 1). In order to compensate for secondary effects, such as contamination of the mixture, in the measurement, the conductance R _p of the measuring device is measured by the second sensor element 4. Conductance is the resistance of the dielectric and therefore the capacitor
Representing a loss of C _p. Thus the measurement of the capacitance of the capacitor C _p is computationally or circuit technically correct.
If the measuring device has only one sensor element,
Capacitance and conductance R _p of the capacitor C _p is sequentially measured by the sensor element.

The sensor elements 3 and 4 are made from a nickel-iron alloy,
It has a thin wall thickness. This material is the wire 7 to the measuring circuit 1
This has the advantage that the contact connection with the substrate is made well. One wire 7 is welded to each of the sensor elements 3 and 4,
Each is brazed to the measuring circuit 1. In this way, the sensor element 3 or 4 becomes an electrical connection between the capacitor _Cp and the measuring circuit 1. Due to the thin wall of the sensor element 3, good heat conduction to the temperature sensor 8 is achieved. Therefore,
In detecting the alcohol content, the dependence of the relative dielectric constant ε _r is taken into account (see FIG. 6).

The sensor elements 3 and 4 have the same configuration. The two sensor elements have approximately the same distance from the casing 2. These sensor elements are attached to the casing 2
Electrically and thermally insulated from

The casing 2 is manufactured from aluminum by die casting. The mixture can be very invasive chemically,
The casing 2 is coated with copper or nickel. As a result, the casing 2 together with the bottom plate 9 exhibits an effective shielding action against electromagnetic fields.

In the measuring device of FIG. 1, the capacitance of the capacitor C _p and
The conductance R _p of a measuring device having two sensor elements 3 and 4 is detected. Depending on the application, a measuring device having only one sensor element can also be used for detecting the alcohol content of the mixture. In this case, the capacitance of the capacitor C _p and the conductance R _p
Is measured continuously over time. The measuring circuit 1 can be connected to each of the two measuring devices.

The measuring circuit 1 (FIG. 2) comprises an oscillator 20 and a control circuit 30 connected before the oscillator. Oscillator 20 is comparator 21
And a RC network, and a comparator is connected to the RC network. The output side 22 of the comparator 21 to the positive input of the comparator via a resistor R ₁ (P input), and is also fed back coupled through a resistor R ₀ to the negative input side (N input).

Output 22 is also connected to a microcontroller not shown in FIG. There, the measurement results are evaluated. That is, the detection of the capacitance and the relative dielectric constant epsilon _r of the capacitor C _p, and thus the alcohol content of the mixture is carried out to detect.

Measuring device represented by a capacitor C _p and the conductance R _p is part of the RC network. The measuring device is connected to the N input side of the comparator 21. The measuring device is connected to ground via a coupling capacitor C _k connected in series. Another capacitor in parallel with the measuring device
C _p0 is connected to ground.

The intermediate tap of the voltage divider is connected to the P input side of the comparator 21. The voltage divider of the supply voltage U ₀ via the resistor R _2a, and is connected to ground via a resistor R _2b.

The control circuit 30 comprises resistors R _3a , R _3b , R _4a , R _4b and two gates 31 and 32. The two gates have tri-state outputs 314, 315, 324, 325. Gates 31 and 32 are controlled independently of each other by the microcontroller via enable inputs 311 to 321. Enable input side 31
If 1 and 321 are at a logic "high" level, inputs 312, 31
3, 322, 323 logic level on output side 314, 315, 324, 325
(The gate is enabled). When the enable input 311 or 321 is at a logic "low" level, the gates 21-32 are disabled (disabled) and the output impedance of the gate outputs 314 and 315-324 and 325 is in a high resistance state.

The control circuit 30 has four different states depending on the control of the two enable inputs 311 and 321. That is, four different control voltages are formed. In the first state, gate 31 is disabled and gate 32 is enabled. In the second state, the two gates are disabled. In the third state, gate 32 is disabled and gate 31
Is enabled. In the fourth state, two gates are enabled.

Outputs 314, 315, 324, 325 of gates 31 and 32 are resistors
It is connected to the P input side via R _3a , R _3b , R4a, R4b.
For example, if gates 31 and 32 are disabled (FIG. 2), the high resistance output impedance acts like a no-load condition at this point. FIG. 3A is obtained as an equivalent circuit. The resistors R _3a , R _3b , R _4a and R _4b have no effect on the measuring circuit 1 in this state. Simply resistance R _2a , R
_2b only determines the input voltage U + at the P input of the comparator 21.

For example, if gates 31 and 32 are enabled (FIG. 2),
The input voltage is conducted to the output side of the gate. That is,
A "high" level appears directly at outputs 314 and 324, and a "low" level appears directly at outputs 315 and 325.

The equivalent circuit for this situation is shown in FIG. The internal resistance of the gate 31 or 32 connected in series with the resistors R _3a , R _3b , R _4a , R _4b respectively can be neglected. This is because the resistors R _3a , R _3b , R _4a , and R _4b are selected to be much larger than the internal resistance. This causes the supply voltage U ₀ to appear at the outputs 314 and 324, and the ground is directly connected to the outputs 315 and 325. In this state, the input voltage U ₊ (control voltage) at the P input side of the comparator 21 is
It is determined by three voltage dividers connected in parallel by resistors R _2a , R _2b , R _3a , R _3b , R _4a and R _4b .

The resistances R _2a , R _2b , R _3a , R _3b , R _4a , and R _4b are the same in pairs. That is, R _2a = R _2b , R _3a = R _3b , R _4a
= _R4b . This ensures that the same operating point is present at the P input of the comparator 21 in each state. That is, in each state, the supply voltage is applied to the P input side.
There is a symmetric average voltage potential between U ₀ and ground. In this embodiment, the resistances R _2a = R _2b = R _3a = R _3b = R _4a = R _4b
200Ω. Thus, the operating point of the comparator 21 is
If the supply voltage is U ₀ = + 5V, it is about + 2.5V.

Resistance R1 is about 200Ω and resistance _R0 is about 680Ω. Gates 31 and 32 are implemented with National Semiconductor's 74ACT245 bus driver. As the comparator 21, a module element CF72220 manufactured by TI is used. The measurement circuit 1 is an application specific integrated circuit (ASIC = Application Specif
ic integrated circuit).

The charge and discharge of the capacitors C _p and C _p0 cause the comparator 21
Is delayed via a feedback path and supplied to its two P and N inputs. As a result, the oscillator 20 oscillates at the oscillator frequency f or the period T, and its transmission characteristic curve has a hysteresis curve.

In order to decouple the measuring device in terms of DC voltage and to set the operating point symmetrically with respect to the P input side, the coupling capacitor C _k is connected in series with the parallel circuit of the capacitor C _p and the conductance R _p. Connected to The capacitance of the capacitor C _p is between about 6pF and 120 pF. This capacitance depends on the alcohol content of the mixture. The loss resistance (expressed here as conductance R _p ) is approximately
Greater than 500Ω. The capacitance of the coupling capacitor C _k is about 100 nF. Coupling capacitor C
_k capacitance is much larger than the capacitance of the capacitor C _p, from the two capacitors C _k and C _p are connected in series, the coupling capacitor C _k is ignored in the detection of the capacitance of the capacitor C _p be able to.

The control voltage at the P input of the comparator 21 oscillates symmetrically about the operating point and depends on different states of the control circuit 30. The following equation holds for the control voltage.

Where U _{+ n} is the control voltage at the P input side, and _Ria and _Rib are i
= 2 represents R _2a and R _2b , correspondingly i =
The same is true for 3 and 4.

When the control circuit 30 takes the second, third or fourth state for the states n = 2, 3, and 4, equation (1) represents the control voltage on the P input side. α _n is a constant, and in the above-described resistor selection configuration, α ₂ = 1/3 in the second state, α ₃ = 1/5 in the _third state, and α ₄ = 1/7.

Input voltage U of N input side _- it is represented by the differential equation.

_{U a = U - · (1} + R 0 / R p) + dU - / dt · R 0 · C p (2) except claim dU _- / dt input voltage U _- represents the time derivative of.

It is assumed that the operating point of the measuring circuit 1 is adjusted by a voltage divider on the P input side, the output voltage U _a is symmetrical with respect to the operating point, and can take only two values of the output voltage deviation _a. . The following equation is true for the output voltage U _a at the operating point.

Here, m = 1, 2,... (Integer; number of cycles), T = cycle period, and t = time.

The equation (1), for each state of the control circuit 30, the control voltage of the cycle period T _n in the P input U ₊ is obtained for the following equation.

The input voltage U of N input _- for, However, an _{R = R 0 ‖R p = R} 0 · R p / (R 0 + R p), -
Is the input voltage deviation on the N input side.

Input voltage U _- oscillates the oscillator frequency f, in the cycle period T in accordance with the hysteresis curve. In this case, oscillation occurs symmetrically with respect to the operating point by two voltage values (switch voltages) given by Expression (5).

Travel delay time of the comparator 21, ie considering the time required for the signal to pass from the input side of the comparator to the output side 22, the formula (5) and thus the input voltage U _-
Are as follows.

_{U - (t = T / 2} -L) = U + (t = T / 2-L) = α n · U a / 2
(6) U ₋ (t = 0) = − U ₋ (t = T / 2) (7) Equation (6) represents the switch voltage already achieved earlier than the switching time by the running delay time L. Equation (7), the input voltage U _- are considering that not a switch voltage at time t = 0, in the voltage reaches the running delay time L beyond this voltage. Considering the peripheral conditions of the equation (5), this equation can be solved according to the cycle period T. State n = 2,3,4
, The next cycle period _Tn is obtained.

T _n = 2 (L + R · C _p · 1n (2 / (1−α _n · (R + R _p ) / R
_p · exp (−L / (R · C _p )))) (8) In each state of the control circuit 30, another oscillator frequency f, another period Tn, and another hysteresis curve are obtained. Equation (8 to the three unknown parameters, namely. Other parameters including conductance Rp and travel delay time L belongs capacitance to be measured of the capacitor C _p, it is determined .3 unknowns are all known This requires three different equations within one measuring cycle, which regulates the three states of the control circuit 30 via the microcontroller and in each state the period T _n
Is measured.

From Expression (8), the output voltage Ua of the comparator 21 does not affect the period Tn. Therefore, the comparator 21
Of the output voltage Ua and the output impedance are not important. Similarly, variations in temperature, time and supply voltage U0 have no effect.

Based on the tolerances of the components, the running delay time L of the signal from the input of the comparator 21 to its output can vary. Since the running delay time L is considered in each measurement cycle, the capacitor Cp and the conductance Rp are determined by the comparator 21.
Can be detected almost independently of the tolerance of the constituent elements. Therefore, the measurement circuit 1 uses an inexpensive comparator 21,
Can be used without sacrificing accuracy.

This measuring circuit 1 measures a small capacitance. Since the charging and discharging time of such a capacitor is very short, the oscillator 20 oscillates at a high oscillation frequency. This would also cause the need for fast and expensive components. Since the oscillator frequency f is limited by the finite running delay time L of the comparator 21, the capacitor Cp
Can be accurately measured only when the oscillator frequency f is reduced. For this purpose, a known capacitor Cp0 is connected in parallel to the capacitor Cp to be detected. This increases the capacitance to be measured. That is, the capacitances of the capacitors Cp and Cp0 are added to each other. The charge / discharge time also increases. Therefore, in the equation (8), the capacitance of the capacitor Cp is calculated as follows:
Must be replaced by a zero capacitance.

The capacitance of the capacitor Cp0 is about 30 pF in this embodiment. Therefore, the capacitance to be detected is about
It is between 36pF and 150pF. In order to obtain the capacitance of the capacitor Cp, the capacitance of the capacitor Cp0 is taken into account, for example by a microcontroller, when evaluating the measurement results.

Another embodiment of the present invention is shown in FIG. In FIG. 4, the same reference numerals are given to parts already described based on FIGS.

A control circuit 30A including switches S1, S2, S3 and a resistor voltage divider including resistors R5, R6, R7, R8, R9 is connected to an oscillator 20A via a P input side and an output side of a comparator 21. The measuring device represented by the capacitor Cp and the conductance Rp is connected to the N input of the comparator 21. Measurement device via the coupling capacitor C _k which are connected in series is connected to ground. The output 22 of the comparator 21 is feedback-coupled to the N input via a resistor R0 and to the P input of the comparator via a control circuit 30A.

Switches S1 to S3 are controlled by a microcontroller not shown in FIG. Depending on the positions of the switches S1 to S3, another control voltage is supplied to the P input side of the oscillator 20A by the voltage dividing ratio of the resistive voltage divider. The control voltage is oscillator 20
Determine the hysteresis curve of A, and thus the oscillator frequency f to be adjusted. Similar to equation (8), three unknown parameters must be determined here as well. To this end, the three states are adjusted by means of different control voltages via the microcontroller, and in each state the period period T and thus the oscillator frequency are measured.

Instead of using tri-state gates 31 and 32 in the control circuit, in this embodiment the switches S1 to S3 are realized as semiconductor switches. The conduction resistance of the semiconductor switch is much smaller than the input impedance of the comparator 21 and therefore requires no attention. The parasitic capacitance of the switches S1 to S3 is reduced to a negligible value by integrating the entire measurement circuit 1 as ASTC. Similarly, ASTC (not shown in FIG. 4) includes components necessary for measuring the period Tn, a counter, a logic circuit controlled by a microcontroller, a crystal oscillator, and a temperature measurement device for measuring the temperature of an air-fuel mixture. The measuring oscillator and the interface circuit for control and data transmission are integrated as a switch circuit.

In addition to the technical fields mentioned above, the measuring device according to the invention can be applied in the area of capacitive sensors. The measuring circuit 1 includes, for example, a chemical sensor for analyzing a fluid containing two components, a filling state measuring device, a distance detector, a pressure sensor,
It can also be used for touch sensors, or high-speed thermometers with fluids with known dielectrics.

The concept "conductance" used above has the same meaning as the concept "loss resistance".

Claims (7)

(57) [Claims] 1. A measuring device for detecting the alcohol content of an air-fuel mixture by measuring the capacitance and conductance of a condenser, wherein the condenser is formed by a first part of a wall of a casing (2). The measuring device is provided with a measuring circuit (1), which is constituted by an electrode and a sensor element (3) exposed to the flow of the air-fuel mixture. A measuring instrument that is electrically connected to the oscillator (20) and further has an oscillator (20), and the capacitance of the capacitor (2, 3) is determined from the frequency of the oscillator. (P) is connected to a control circuit (30), from which at least three different control voltages are supplied, and the output of the oscillator (20) is connected to an evaluation circuit. The evaluation circuit measures the oscillator frequency adjusted for each of the control voltages, and determines the capacitance (Cp) and conductance (Rp) of the capacitors (2, 3) from the measured oscillator frequency. A measuring instrument for detecting the alcohol content of an air-fuel mixture. 2. The measuring device according to claim 1, wherein the evaluation circuit connected to the output of the oscillator is a microcontroller. 3. The evaluation circuit connected to the output side of the oscillator (20) is a microcontroller, by which the delay time (L) of the oscillator is obtained from the measured oscillator frequency. The measuring device described in the item. 4. The measuring device according to claim 1, wherein the oscillator (20) has a comparator (21) connected to the RC network. 5. The control circuit (30) has at least one gate (31, 32) having a tri-state output (314, 315, 324, 325) and connected to a resistor network. The measuring instrument according to claim 1, wherein the measuring instrument is used. 6. A method for detecting the alcohol content of an air-fuel mixture by measuring the capacitance and conductance of a capacitor, the capacitor comprising a first electrode formed by a part of a wall of a casing (2). And a second electrode formed by the sensor element (3) exposed to the flow of the air-fuel mixture. The measuring device is provided with a measuring circuit (1), and the measuring circuit is provided with the capacitors (2, 3) electrically connected to the control circuit (30) and further comprising an oscillator (20), wherein the control circuit (30) determines at least three different sizes of the capacitors (2, 3) from the frequency of the oscillator. And providing said control voltage to an oscillator (20) to form at least three oscillator frequencies by changing said control voltage; Determining a corresponding number of parameters of the measuring circuit (1) from the oscillator frequency of the mixture. 7. The method according to claim 6, wherein the capacitance and the conductance of the capacitor are detected from the measured oscillator frequency, and the relative permittivity (ε _r ) of the air-fuel mixture is detected from the capacitance and the conductance.